Various digital data communication systems are well known in the art. One such exemplary system is described in U.S. Pat. No. 3,979,719 to Tooley et al. Tooley discloses a multiple block binary synchronous duplex communication system in which data is transmitted in blocks including an identification code and an error checking code. Received data is checked for errors and when an error is found, the transmitting station is provided information via a return path identifying the incorrectly received data. The transmitting station retransmits the incorrectly received data when the transmitter becomes available.